Conventionally there is known a servo motor control device that implements model following control. For example, Japanese Patent Unexamined Publication No. 8-168280 (published on Jun. 25, 1996) discloses a control device that, in control of a model (a servo motor as a reference model) used to implement model following control on a servo motor, implements proportional-integral-derivative (PID) control such that a position and a velocity of a model follow an externally input position command value and an externally input velocity command value.
However, in the conventional technique, in the case that the torque output from the model exceeds the maximum torque that can be output from the servo motor, a feedback controller cannot follow an orbit output from the model, so that problems such as overshoot and hunting occur.
When the torque output from the model exceeds the maximum torque that can be output from the servo motor, the servo motor cannot follow a torque value of the model. Therefore, in order to limit the torque output from the model, it is conceivable that a torque limiter is provided such that a torque command value provided to the servo motor does not exceed the maximum torque that can be output from the servo motor. However, in the configuration, although the torque output from the model is limited, position followability and velocity followability are not ensured due to the torque limitation, so that a deviation increases. Therefore, for example, even if the position of the servo motor comes close to target position to decrease the velocity command, the model output torque is kept large in order to solve the increased deviation, and the overshoot and the hunting occur in the orbit output from the model. Resultantly, there is a possibility that the overshoot and the hunting occur in the real control target (for example, the servo motor).
Accordingly, in the conventional technique of controlling the model using the PID control, when a user uses the servo motor control device that implements the model following control, it is necessary that the user provide the position command value while being conscious of the maximum torque that can be output from the servo motor.